RIGGING in 2.79b ~ IK handles cause hands/feet to detatch from the body, alternatives? (Foot rigging)
Parent the foot to the IK controller (therefore now it's unparented from the lowerleg, as explained in your tutorials).

Give your lowerleg a IK constraint with the controller as Target:

Now to make sure that the foot sticks to the lowerleg, give the foot a Copy Location constraint, with the lowerleg as Target, a Head/Tail value of 1, and a Space setting &gt; World Space / World Space.

file here: